Ultrasonic sensing techniques have become widely acceptable for use in ranging systems for determining the presence of and distance to an object. In a conventional ultrasonic ranging system, an ultrasonic transducer is employed which converts electrical signal pulses into mechanical motion. In turn, the mechanical motion creates ultrasonic sound waves that are transmitted through the atmosphere in a desired direction. Provided there is a target in its path, the sound waves reflect off the target and the reflected sound waves travel back to the ultrasonic transducer. The reflected sound waves, also referred to as the echo waves, mechanically deflect the ultrasonic transducer and, in response, a low voltage pulsed signal is generated. Since the speed of travel of the sound waves at a given temperature remains relatively fixed, the distance to the target is determined by measuring the time period between the transmitted and received signal pulses, and computing the distance as a function of the time period and the sound wave speed. This determined distance can be calculated directly or through a pattern recognition algorithm.
1. Transducer Ringing
Ultrasonic transducers can be used both to send and to receive ultrasonic waves. However, commercially available ultrasonic transducers, such as the Murata MA40S4R/S, due to their high quality factor Q continue to emit ultrasound even after all power to the transducer has been turned off. As a result, residual electrical oscillations at the transducer terminals deteriorate and mask weak received signals. This is known as ringing and is similar to the sound that a bell continues to emit after it has been struck.
This ringing prevents the use of such a transducer as a receiver until the ringing has subsided to the point that the received waves exceed the magnitude of the waves being emitted. Such transducers effectively cannot sense a reflection from a target closer than some particular distance from the transducer depending on the amount of ringing, which for a standard MuRata transducer may be as much as about 30 cm. Depending on the particular system design, an occupant can get quite close to the transducers, sometimes as close as 10 cm. Thus, when it is necessary to sense the presence of an object closer than the ringing zone, ultrasonic systems heretofore have required that the transducers be used in pairs, one for sending and another for receiving. The requirement to use pairs of transducers increases the cost of the system and when the ultrasonic system is arranged in a vehicle, it would occupy valuable real estate in the vehicle.
2. Clicking
The transmitted and received ultrasonic sound waves are similar to audible sound waves, except the ultrasonic frequencies are generally much higher and therefore exceed the audible frequency range for human beings. Accordingly, human beings are generally unable to hear the radiated ultrasonic sound waves generated by the ultrasonic transducer. In many conventional applications, the ultrasonic ranging system is generally considered to be a quiet operating device. However, in practice, it is recognized that an ultrasonic transducer creates undesired audible waves as a side effect when transmitting ultrasonic sound waves, particularly at certain strength levels. The presence of audible sound is even more noticeable where a high strength signal is required. It has been discovered that these undesirable audible sound waves generally provide a noticeable audible “click” sounding noise which, in the past, has generally been considered acceptable for some applications. However, the audible “click” noise generated by an ultrasonic transducer can be annoying when used in certain environments, such as inside the passenger compartment of a vehicle or other places where humans or other animals can be present. In particular, this “click” becomes more pronounced when the range of the transducer is increased by increasing the amplitude of the ultrasonic waves.
The “click” is present in both piezoelectric electrostatic transducers such as manufactured by Polaroid and in solid piezoelectric transducers such as manufactured by MuRata. It is noteworthy that in the Polaroid case, since the device has a low Q, nearly the full amplitude of the ultrasound is achieved on the first cycle and thus a burst of waves naturally has essentially a square wave envelop. In contrast, the higher Q MuRata transducers require a significant number of cycles to reach full amplitude and to die off after the driving pulse has been removed and thus, even though the driving circuit puts out a square wave envelop, the transducer appears to be modulated by a sine wave. As a result, the forced modulation as described in U.S. Pat. No. 06,243,323 and U.S. Pat. No. 06,202,034 may be practiced when using Polaroid type transducers but is not necessary when using MuRata type transducers. Also, since this fact has been well known for a long time, there is nothing believed to be novel about modulating the output of an ultrasonic transducer with a “smooth modulation envelop” as claimed in the '323 and '034 patents.
Of even greater significance, the “click” is present in both the Polaroid and MuRata transducers and thus, the existence of a “smooth modulation envelop” does not in fact remove the “click” as reported in the '323 and '034 patents. The effect experienced by Li (the '323 patent) is probably merely the result of a reduced total energy of the pulses that are being transmitted.
The cause of the “click” is still not totally understood and is certainly not the “sudden acceleration of the air” as reported in the '323 patent. The acceleration of this air is at a maximum when the ultrasonic wave amplitude is at a maximum. One theory is that the clicking noise is a result of the nonlinear adiabatic air expansion and compression that occurs when the ultrasound pulse is introduced into the atmosphere which it is theorized causes the waves to oscillate about a non-zero level. This non-zero level, or bias, therefore creates a pulse at the repetition rate of the transducer. In support of this theory, it has been found that the clicking amplitude can be reduced for the same total energy per burst by reducing the peak ultrasound amplitude and increasing the number of cycles. Of course, this has the drawback of making it more difficult to differentiate between different closely spaced reflective surfaces. This reduces the resolution of the device when using ultrasound for monitoring the occupancy of a passenger compartment of a vehicle, for example, since it is the pattern of the returned cycles that contains vital information used to categorize, classify, ascertain the identity of and/or identify the occupying item of the seat and to determine its location in the vehicle passenger compartment. If a longer burst of waves is used, then the reflections from different surfaces are blurred and the pattern of reflected waves becomes less distinct reducing the accuracy of the occupant classification and location system.
Occupant sensors are now being used on production automobiles that make use of ultrasonic transducers in a system to locate and identify the occupancy of the front passenger seat of an automobile to suppress deployment of an airbag if the seat is empty, if a rear facing child seat is present or if an occupant is out-of-position. Out-of-position is typically considered a situation when the occupant is so close to the airbag that the deployment is likely to cause greater injury to the occupant than its non-deployment.
Thus, in addition to a method to reduce this ringing so as to enable a single transducer to be used both for sending and receiving from targets as close as about 10 cm, there is also a need to eliminate the audible clicking noise.